Phonon-Assisted Anti-Stokes Lasing in ZnTe Nanoribbons

Bimetallic Ag125Cu8 Nanocluster, Structure Determination, and Nonlinear Optical Properties

The atomic precision of the subnanometer nanoclusters has provided sound proof on the structural correlation of metal complexes and larger-sized metal nanoparticles. Herein, we report the synthesis, crystallography, structural characterization, electrochemistry, and optical properties of a 133-atom intermetallic nanocluster protected by 57 thiolates (3-methylbenzenethiol, abbreviated as m-MBTH) and 3 chlorides, with the formula of Ag125Cu8(m-MBT)57Cl3. This is the largest Ag-Cu bimetallic cluster ever reported. Crystallographic analysis revealed that the nanocluster has a three-layer concentric core-shell structure, Ag7@Ag47@Ag71Cu8S57Cl3, and the Ag54 metal kernel adopts a D5h symmetry. The nuclei number is between that of the previously reported large silver cluster [Ag136(SR)64Cl3Ag0.45]- and the large silver-rich cluster Au130-xAgx(SR)55 (x = 98). All these three clusters bear a similar metallic core structure, while the main structural difference lies in the shell motif structures. Electron counting revealed an open electron shell with 73 delocalized electrons, which was verified by the electron paramagnetic resonance analysis. The DPV electrochemical measurement indicates a multielectron state quantization double-layer charging shape and single-electron sequential charging and discharging characteristic of the AgCu alloy cluster. In addition, the open-hole Z-scan test reveals the nonlinear optical absorption (2-3 optical absorption in the NIR-II/III region) of Ag125Cu8 nanoclusters.

Photophysics and its application in photon upconversion

Photoluminescence (PL) upconversion is a phenomenon involving light-matter interaction, where the energy of the emitted photons is higher than that of the incident photons. PL upconversion has promising applications in optoelectronic devices, displays, photovoltaics, imaging, diagnosis and treatment. In this review, we summarize the mechanism of PL upconversion and ultrafast PL physical processes. In particular, we highlight the advances in laser cooling, biological imaging, volumetric displays and photonics.

A Bifunctional Optoelectronic Device for Photodetection and Photoluminescence Switching Based on Graphene/ZnTe/Graphene van der Waals Heterostructures

Controlling the dynamic processes, such as generation, separation, transport, and recombination, of photoexcited carriers in a semiconductor is foundational in the design of various devices for optoelectronic applications. One may imagine that if different processes can be manipulated in one single device and thus generate useful signals, a multifunctional device can be realized, and the toolbox for integrated optoelectronics will be expanded. Here, we revealed that in a graphene/ZnTe/graphene van der Waals (vdW) heterostructure, the carriers can be generated by illumination from visible to infrared frequencies, and thus, the detected spectrum range extends to the communication band, well beyond the band gap of ZnTe (2.26 eV). More importantly, we are able to control the competition between separation and recombination of the photoexcited carriers by an electric bias along the thickness-defined channel of the ZnTe flake: as the bias increases, the photodetecting performance, e.g. response speed and photocurrent, are improved due to the efficient separation of carriers; synchronously, the photoluminescence (PL) intensity decreases and even switches off due to the suppressed recombination process. The ZnTe-based vdW heterostructure device thus integrates both photodetection and PL switching functions by manipulating the generation, separation, transport, and recombination of carriers, which may inspire the design of the next generation of miniaturized optoelectronic devices based on the vdW heterostructures made by various thin flakes.

Phonon-assisted upconversion photoluminescence of monolayer MoS2 at elevated temperatures

Upconversion photoluminescence (UPL) lies at the heart of optical refrigeration and energy harvesting. Monolayer transition metal dichalcogenides (TMDCs) have been identified as an excellent platform with robust phonon-exciton coupling for studying the phonon-assisted UPL process. Herein, we investigate the multiphonon-assisted UPL emission in monolayer MoS2 at elevated temperatures and the temperature-dependent phonon contributions in the UPL process. When temperature goes up from 295 K to 460 K, the enhancement of the integrated UPL intensity is demonstrated due to the increased phonon population and the reduced phonon numbers involved in the UPL process. Our findings reveal the underlying mechanism of phonon-assisted UPL at high temperatures, and pave the way for the applications of photon upconversion in display, nanoscale thermometry, anti-Stokes energy harvesting, and optical refrigeration.

Ultrafast Antisolvent Growth of Single-Crystal CsPbBr3 Microcavity for Whispering-Gallery-Mode Lasing

In recent years, all-inorganic cesium lead bromide (CsPbBr₃) perovskites have garnered considerable attention for their prospective applications in green photonics and optoelectronic devices. However, the development of efficient and economical methods to obtain high-quality micron-sized single-crystalline CsPbBr₃ microplatelets (MPs) has become a challenge. Here, we report the synthesis of CsPbBr₃ MPs on Si/SiO₂ substrate by optimizing the ultrafast antisolvent method (FAS). This technique is able to produce well-dispersed, uniformly sized, and morphologically regular tetragonal phase single crystals, which can give strong green emission at room temperature, with excellent stability and excitonic character. Moreover, the crystals demonstrated lasing with a whispering gallery mode with a low threshold. These results suggest that the single-crystalline CsPbBr₃ MPs synthesized by this method are of high optical quality, holding vast potential for future applications in photonic devices.

Light Amplification in Fe-Doped CsPbBr3 Crystal Microwire Excited by Continuous-Wave Laser

Electrically pumped halide perovskite laser diodes remain unexplored, and it is widely acknowledged that continuous-wave (CW) lasing will be a crucial step. Here, we demonstrate room-temperature amplified spontaneous emission of Fe-doped CsPbBr₃ crystal microwire excited by a CW laser. Temperature-dependent photoluminescence spectra indicate that the Fe dopant forms a shallow level trap states near the band edge of the lightly doped CsPbBr₃ microcrystal. Pump intensity-dependent time-resolved PL spectra show that the introduced Fe dopant level makes the electron more stable in excited states, suitable for the population inversion. The emission peak intensity of the lightly Fe-doped microwire increases nonlinearly above a threshold of 12.3 kW/cm² under CW laser excitation, indicating a significant light amplification. Under high excitation, the uniform crystal structure and surface outcoupling in Fe-doped perovskite crystal microwires enhanced the spontaneous emission. These results reveal the considerable promise of Fe-doped perovskite crystal microwires toward low-cost, high-performance, room-temperature electrical pumping perovskite lasers.

Defect-Dependent Nonlinear Absorption in the Lead-Free Double-Perovskite Cs2AgBiBr6

Lead-free perovskites have attracted increasing attention because they can address the toxicity and instability problems inherent to lead-halide perovskites. Furthermore, the nonlinear optical (NLO) properties of lead-free perovskites are rarely explored. Herein, we report significant NLO responses and defect-dependent NLO behavior of Cs₂AgBiBr₆. Specifically, a thin film of pristine Cs₂AgBiBr₆ exhibits strong reverse saturable absorption (RSA), while a film of Cs₂AgBiBr₆ with defects (denoted as Cs₂AgBiBr₆(D)) shows saturable absorption (SA). The nonlinear absorption coefficients are ca. 4.0 × 10⁴ cm GW^-1 (515 nm laser excitation) and 2.6 × 10⁴ cm GW^-1 (800 nm laser excitation) for Cs₂AgBiBr₆ and -2.0 × 10⁴ cm GW^-1 (515 nm laser excitation) and -7.1 × 10³ cm GW^-1 (800 nm laser excitation) for Cs₂AgBiBr₆(D). The optical limiting threshold of Cs₂AgBiBr₆ is 8.1 × 10^-4 J cm^-2 (515 nm laser excitation). The samples show excellent long-term performance stability in air. The RSA of pristine Cs₂AgBiBr₆ correlates with excited-state absorption (515 nm laser excitation) and excited-state absorption following two-photon absorption (800 nm laser excitation), while the defects in Cs₂AgBiBr₆(D) strengthen the ground-state depletion and Pauli blocking, resulting in SA.

Size-Dependent Phonon-Assisted Anti-Stokes Photoluminescence in Nanocrystals of Organometal Perovskites

Anti-Stokes photoluminescence (ASPL), which is an up-conversion phonon-assisted process of the radiative recombination of photoexcited charge carriers, was investigated in methylammonium lead bromide (MALB) perovskite nanocrystals (NCs) with mean sizes that varied from about 6 to 120 nm. The structure properties of the MALB NCs were investigated by means of the scanning and transmission electron microscopy, X-ray diffraction and Raman spectroscopy. ASPL spectra of MALB NCs were measured under near-resonant laser excitation with a photon energy of 2.33 eV and they were compared with the results of the photoluminescence (PL) measurements under non-resonant excitation at 3.06 eV to reveal a contribution of phonon-assisted processes in ASPL. MALB NCs with a mean size of about 6 nm were found to demonstrate the most efficient ASPL, which is explained by an enhanced contribution of the phonon absorption process during the photoexcitation of small NCs. The obtained results can be useful for the application of nanocrystalline organometal perovskites in optoelectronic and all-optical solid-state cooling devices.

Ultrabroadband Tuning and Fine Structure of Emission Spectra in Lanthanide Er-Doped ZnSe Nanosheets for Display and Temperature Sensing

Realizing multicolored luminescence in two-dimensional (2D) nanomaterials would afford potential for a range of next-generation nanoscale optoelectronic devices. Moreover, combining fine structured spectral line emission and detection may further enrich the studies and applications of functional nanomaterials. Herein, a lanthanide doping strategy has been utilized for the synthesis of 2D ZnSe:Er³⁺ nanosheets to achieve fine-structured, multicolor luminescence spectra. Simultaneous upconversion and downconversion emission is realized, which can cover an ultrabroadband optical range, from ultraviolet through visible to the near-infrared region. By investigating the low-temperature fine structure of emission spectra at 4 K, we have observed an abundance of sublevel electronic energy transitions, elucidating the electronic structure of Er³⁺ ions in the 2D ZnSe nanosheet. As the temperature is varied, these nanosheets exhibit tunable multicolored luminescence under 980 and 365 nm excitation. Utilizing the distinct sublevel transitions of Er³⁺ ions, the developed 2D ZnSe:Er³⁺ optical temperature sensor shows high absolute (15.23% K^-1) and relative sensitivity (8.61% K^-1), which is superior to conventional Er³⁺-activated upconversion luminescent nanothermometers. These findings imply that Er³⁺-doped ZnSe nanomaterials with direct and wide band gap have the potential for applications in future low-dimensional photonic and sensing devices at the 2D limit.

Realizing Saturable Absorption and Reverse Saturable Absorption in a PEDOT:PSS Film via Electrical Modulation

Electrical tuning of the nonlinear absorption of materials has promising application potential, while studies remain rare. In this work, we show that the third-order nonlinear absorption of poly(3,4-ethylenedioxythiophene) chemically doped with poly(styrene sulfonic acid) [PEDOT:PSS] can be effectively modulated by external voltage. The nonlinear absorption of the film can be varied between reverse saturable absorption (RSA) and saturable absorption (SA) via voltage control with laser excitation at 800 nm, and the corresponding nonlinear absorption coefficient can be tuned in the range -1606 ± 73 to 521 ± 9 cm GW^-1. The doping level and energy structure of PEDOT are modulated with different voltages. The undoped film affords two-photon absorption and accordingly the RSA response. A moderately doped sample has two polaron levels, and Pauli blocking associated with these two polaron levels results in SA. The bipolaron level in heavily doped PEDOT leads to excited-state absorption and therefore RSA behavior. The approach reported here can be applied to other semiconductors and is a convenient, effective, and promising method for the electrical tuning of the optical nonlinearity.

Electro- and photon-induced cooling in BNT-BT-SBET relaxors with in situ optical temperature sensing

A novel lead-free luminescent ferroelectric (FE) ceramic, ${{\rm Bi}_{0.5}}{{\rm Na}_{0.5}}{{\rm TiO}_3} {-} {0.{06\; \rm BaTiO}_3} {-} {0.{055\;\rm Sr}_{0.7}}{{\rm Bi}_{0.18}}{{\rm Er}_{0.02 \,\square\, 0.1}}$Bi_0.5Na_0.5TiO₃-0.06BaTiO₃-0.055Sr_0.7Bi_0.18Er_0.02◻0.1${{\rm TiO}_3}$TiO₃ (BNT-BT-SBET), is developed with an adiabatic temperature change ($\Delta T$ΔT) of 0.7 K under an electric field ($E$E) of 60 kV/cm at room temperature, an anti-Stokes fluorescence cooling, and a maximum optical $T$T sensitivity of ${0.0055}\;{{\rm K}^{ - 1}}$0.0055K^-1 at 522 K. Interestingly, the electrocaloric response reaches a saturation at permittivity shoulder $T$T of 100°C; meanwhile, the maximized emission intensity of $^2{{\rm H}_{11/2}}{ \to ^4}{{\rm I}_{15/2}}$²H_11/2→⁴I_15/2 occurs. $T$T- and $E$E-tunable enhancement of $^2{{\rm H}_{11/2}}{ \to ^4}{{\rm I}_{15/2}}$²H_11/2→⁴I_15/2 emission intensity is due to the population inversion from the $^4{{\rm S}_{3/2}}$⁴S_3/2 to $^2{{\rm H}_{11/2}}$²H_11/2 states caused by an incoherent regime consisting of FE phase and polar nanoregions in a relaxor matrix.

Unveiling lasing mechanism in CsPbBr3 microsphere cavities

Recently, the light-matter interaction of perovskite microcavities has been widely explored for its great potential in low-threshold lasing devices. However, the mechanism of perovskite lasing remains unclear to date. In this study, we demonstrated high-quality single-mode excitonic lasing in CsPbBr3 microspheres, providing an ideal platform to study the underlying physics of lasing behavior. We show that the lasing mechanism shifts from the exciton-exciton scattering to the exciton-phonon scattering with the increase in temperature from 77 to 300 K, which was verified by temperature-dependent photoluminescence (PL), time-resolved photoluminescence (TRPL) as well as temperature-dependent Raman spectroscopy. Furthermore, by analyzing PL line width broadening with varied temperatures, we found that two different phonon modes were involved in the exciton-phonon scattering process. The scattering from the low-energy phonon (∼8.6 meV) is the dominant source of exciton-phonon coupling in the intermediate temperature range (77 to 230 K), while the high-energy phonon (∼15.3 meV) dominates from 230 K to room temperature. These results confirm the lasing mechanism in such perovskite-based micro/nano-cavities and significantly influence the development of future low-threshold lasers.

An Ultrabroadband Mid-Infrared Pulsed Optical Switch Employing Solution-Processed Bismuth Oxyselenide

Pulsed lasers operating in the mid-infrared (3-25 µm) are increasingly becoming the light source of choice for a wide range of industrial and scientific applications such as spectroscopy, biomedical research, sensing, imaging, and communication. Up to now, one of the factors limiting the mid-infrared pulsed lasers is the lack of optical switch with a capability of pulse generation, especially for those with wideband response. Here, a semiconductor material of bismuth oxyselenide (Bi₂ O₂ Se) with a facile processibility, constituting an ultrabroadband saturable absorber for the mid-infrared (actually from the near-infrared to mid-infrared: 0.8-5.0 µm) is exhibited. Significantly, it is found that the optical response is associated with a strong nonlinear character, showing picosecond response time and response amplitude up to ≈330.1% at 5.0 µm. Combined with facile processibility and low cost, these solution-processed Bi₂ O₂ Se materials may offer a scalable and printable mid-infrared optical switch to open up the long-sought parameter space which is crucial for the exploitation of compact and high-performance mid-infrared pulsed laser sources.

Reverse Saturable Absorption Induced by Phonon-Assisted Anti-Stokes Processes

In materials showing reverse saturable absorption (RSA), optical transmittance decreases at intense laser irradiation. One approach to application of these materials is to protect the sensors or human eyes from laser damage. To date, research has mainly concentrated on thin films and suspensions of graphite and its nanostructure (including nanotubes, graphene, and graphene oxides), which are mainly used as an optical limiter for nanosecond laser pulses. Moreover, thin individual pieces of semiconductor usually exhibit increased transmittance due to saturable absorption when the laser energy (E_laser ) is higher than the band gap (E_B ). Here, it is shown that indirect gap semiconductor WSe₂ exhibits high RSA on exposure to a femtosecond laser under E_laser > E_B near band gap excitation, which is attributed to the longitudinal optical phonon-assisted anti-Stokes transition by the annihilation of phonons and the absorption of photons. An optical limiting threshold (≈21.6 mJ cm^-2 ) lower than those reported for other optical-limiting materials currently for femtosecond laser at 800 nm is observed.

Laser Refrigeration of Ytterbium-Doped Sodium-Yttrium-Fluoride Nanowires

Sodium yttrium fluoride (β-NaYF₄ ) nanowires (NWs) with a hexagonal crystal structure are synthesized using a low-cost hydrothermal process and are shown to undergo laser refrigeration based on an upconversion process leading to anti-Stokes (blueshifted) photoluminescence. Single-beam laser trapping combined with forward light scattering is used to investigate cryophotonic laser refrigeration of individual NWs through analysis of their local Brownian dynamics.

Laser cooling in solids: advances and prospects

This review discusses the progress and ongoing efforts in optical refrigeration. Optical refrigeration is a process in which phonons are removed from a solid by anti-Stokes fluorescence. The review first summarizes the history of optical refrigeration, noting the success in cooling rare-earth-doped solids to cryogenic temperatures. It then examines in detail a four-level model of rare-earth-based optical refrigeration. This model elucidates the essential roles that the various material parameters, such as the spacing of the energy levels and the radiative quantum efficiency, play in the process of optical refrigeration. The review then describes the experimental techniques for cryogenic optical refrigeration of rare-earth-doped solids employing non-resonant and resonant optical cavities. It then examines the work on laser cooling of semiconductors, emphasizing the differences between optical refrigeration of semiconductors and rare-earth-doped solids and the new challenges and advantages of semiconductors. It then describes the significant experimental results including the observed optical refrigeration of CdS nanostructures. The review concludes by discussing the engineering challenges to the development of practical optical refrigerators, and the potential advantages and uses of these refrigerators.

Phonon-Assisted Population Inversion in Lanthanide-Doped Upconversion Ba2 LaF7 Nanocrystals in Glass-Ceramics

The effective population inversion of ² H_11/2 from ⁴ S_3/2 state of Er³⁺ ions can be achieved through the annihilation of phonons; random lasing action from BLF films embedded with Yb³⁺ /Er³⁺ codoped BLF nanocrystals is demonstrated and high ambient temperature (>433 K) operation lasers with a very low excitation threshold (<530 nJ cm^-2 ) are realized.